1. Field of the Invention
The present invention relates to a signal amplifier used in an automatic gain control amplifier having a wide gain variable width.
2. Description of the Related Art
An automatic gain control amplifier that keeps an output signal level constant with respect to a change in an input signal level is used for various applications. When a circuit of such an automatic gain control amplifier is formed in an IC (integrated circuit), as a method of forming a variable gain amplifier circuit, the following two methods are conceivable.
A first method is a method of converting an input signal into a signal current and amplifying the signal current. A second method is a method of changing a load resistance for determining an amplification degree of an output signal to change an output level. The first method is often applied to analog circuits. Since again is changed stepwise in the second method, the second method is often used in circuits in which MOS transistors are used.
It is possible to relatively easily realize the first method using a bipolar junction transistor (BJT). An example of a basic structure of the bipolar junction transistor is shown in FIG. 8. In FIG. 8, emitters of NPN-type transistors 101 and 102 are connected in common to a current source 103. A control signal voltage Vc is applied between bases of the transistors 101 and 102. When electric currents supplied to collectors of the transistors 101 and 102 are I1 and I2, respectively, and an electric current flowing through the current source 103 is 2·I0, the following relation is established between the currents I1, I2, and I0 and the control signal voltage Vc.
                                                        I              1                        +                          I              2                                =                      2            ·                          I              0                                      ⁢                                  ⁢                              V            C                    =                                                                      kT                  q                                ⁢                                  ln                  ⁡                                      (                                                                  I                        1                                                                    I                        2                                                              )                                                              ⁢                                                          ∴                              I                1                                      =                                                            exp                  ⁡                                      (                                                                  q                        ·                                                  V                          C                                                                    kT                                        )                                                                    1                  +                                      exp                    ⁡                                          (                                                                        q                          ·                                                      V                            C                                                                          kT                                            )                                                                                  ·              2              ·                              I                0                                                                        Equation        ⁢                                  ⁢        1            
As it is evident from this equation, values of the currents I1 and I2 change by a great degree according to the control signal voltage Vc.
In the second method, a normal MOS transistor is used as a switch element. A load resistance used in a voltage-current converting unit on an input side of an amplifier or a current-voltage converting unit on an output side of the amplifier is changed stepwise. A gain changes discretely (digitally or stepwise) rather than changing linearly (analogically).
FIGS. 9A and 9B are diagrams for explaining an example of a variable gain amplifier circuit according to the second method. FIG. 9A is a circuit diagram showing an example of a basic structure of the variable gain amplifier circuit. An input terminal 111, to which an input signal voltage Vin is supplied, is connected to an inverting input terminal (−terminal) of an operational amplifier 113 via a resistor 112 serving as a voltage-current converting unit on an input side. A noninverting input terminal (+terminal) of the operational amplifier 113 is connected to a reference potential, for example, a ground potential. A load resistor circuit 120 serving as a current-voltage converting unit on an output side is connected between the inverting input terminal (−terminal) and an output terminal 114 of the operational amplifier 113. A value (a load resistance Rd) of the load resistor circuit 120 changes stepwise (discretely or digitally). In the load resistor circuit 120, a serial connection circuit of a resistor 121a and a switch 122a, a serial connection circuit of a resistor 121b and a switch 122b, . . . , a serial connection circuit of a resistor 121n and a switch 122n are connected in parallel to one another. The switches 122a, 122b, . . . , 122n of the load resistor circuit 120 are subjected to changeover control according to a changeover control signal (Control-Bit) from the terminal 123 and a resistance of the load resistor circuit 120 changes stepwise. Consequently, as shown in FIG. 9B, a gain changes stepwise (discretely or digitally).
An example of the first method is disclosed in JP-A-9-246897 and an example of the second method is disclosed in JP-A-2003-273674.